STM study of Ag film initial stages growth on a GaN(0001) surface grown by MBE

The adsorption and growth of Ag on the GaN(0001) pseudo 1x1-Ga surface was studied by using a combined scanning tunneling microscopy (STM)-molecular beam epitaxy (MBE) system. Ag shows high mobility and huge diffusion length on the surface, which results in the formation of large monolayer Ag islands at submonolayer coverage. We report the observation of a drastic change of the Ag growth mode on the GaN(0001) surface, from Stranski-Krastanov growth at low Ag flux (similar to0.8 ML/min) to layer-by-layer growth at a high flux (1 ML/sec). Based on this finding a new approach to obtain flat epitaxial Ag film on the GaN(0001) surface, by using high Ag flux, is demonstrated. In addition, by annealing the Ag film-covered GaN(0001) surface we found a novel unreconstructed Ag-terminated GaN(0001)-1 x 1 and explain its structure by a T-1-site adatom model.